1. Field of the Invention
The invention relates to the use and operation of particle accelerator systems. More specifically, the invention relates to techniques for synchronizing signals within a particle accelerator system such that a stable, reliable and predictable output is obtained from the particle accelerator system.
2. Description of the Related Art
Conventional particle accelerator systems serve to manipulate and control atomic and sub-atomic particles through the use of electromagnetic fields. For example, some particle accelerators are used to cause the collision of atomic particles (with one another or with a separate object), so that sub-atomic particles can thereby be obtained. As another example, particle accelerators can be used as part of systems designed to produce high-energy, coherent light sources (e.g., the Free Electron Laser (FEL), which uses an electron beam as its lasing medium).
In accelerator systems operating to create an electron beam having desired properties, electrons can be obtained from a radio-frequency (RF) photocathode electron gun. Such a photocathode electron gun typically requires a pulse of (laser) light to be provided to the cathode at a time that is very precisely determined with respect to the phase of the electromagnetic field inside the resonant cavity of the electron gun. This phase of the field within the cavity is conventionally approximated by using the phase of the RF drive applied to the gun, as will be discussed in more detail below.
Providing the laser pulse to the photocathode at the appropriate time is typically accomplished by driving the source laser with an RF source (i.e., frequency generator) carefully phase-locked to a higher-frequency RF drive being applied to the electron gun (as well as to the accelerator itself). This technique, using common lasers for the purpose just described, provides what is known as a xe2x80x9clock-to-clockxe2x80x9d capability to thereby provide a conventional level of timing accuracy.
FIG. 1 illustrates a known accelerator system 100. In FIG. 1, master RF oscillator 105 generates and supplies a low-frequency signal to seed laser 110. Seed laser 110 may include, for example, a mode-locked Ti3+ doped Sapphire (Ti:S) oscillator operating at 81.6 MHz, thereby providing a train of pulses with a pulse width of around 150 fs (1.5xc3x9710xe2x88x9213 sec). The output of seed laser 110 is fed into amplifier chain 120. Together, the seed laser 110 and amplifier chain 120 form a laser system 125.
The pulses output by laser system 125 are directed, for example by mirror 130, through entrance window 135 of electron gun 140. A photocathode (not shown) within electron gun 140 is thereby stimulated to produce electrons, which are then supplied to Linear Accelerator (LINAC) 145.
Master RF oscillator 105 also generates a high-frequency signal output to high power RF amplifier 150 for use by electron gun 140 and LINAC 145. The electron gun 140 and LINAC 145 may operate in the S-band of the microwave spectrum, generally defined as within a range of 2800-3000 MHz. The high-frequency signal sent to high power RF amplifier 150 may be locked to a multiple of the low-frequency laser signal sent to seed laser 110. In the system of FIG. 1, the high frequency signal may be, for example, at a frequency of 2856 MHz, the 35th harmonic of the low-frequency signal to laser system 125. High power RF amplifier 150 and variable power splitter 155 can be used to manipulate the high-frequency output of master oscillator 105 in terms of power and direction, respectively.
Various difficulties are associated with the operation of conventional accelerator systems such as system 100 shown in FIG. 1. For example, the laser pulse should be provided to the electron gun 140 within a window of approximately 1xc2x0 of phase with respect to the RF field within the electron gun 140. This correspond to about 1 ps of timing jitter and drift between the laser system 125 and the master RF oscillator 105. Because the RF field has a frequency of about 35 times the frequency of the laser system, a lock stability of about 1/35xc2x0 exists on the laser system 125, which is extremely difficult to maintain.
As another example of the shortcomings of system 100, amplifier chain 120 often contributes timing drift to the system 100. Because the desired 1 ps timing window corresponds to as little as 1 part per million of the total time the pulse is propagated between the laser system 125 and the electron gun 140, even a tiny drip in the delay of the pulse through the laser system 125 can significantly degrade the timing of a pulse. This degradation can occur even if the signal applied to seed laser 110 by master RF oscillator 105 starts out perfectly timed with the phase of the electric field within a resonant cavity (not shown) of electron gun 140. For example, a 1% change in atmospheric density can easily provide this much timing drift.
A final example of problems associated with system 100 results from changes in the phase of the RF field inside the resonant cavity of the electron gun 140 relative to the phase of the RF drive supplied to the electron gun 140 by master RF oscillator 105. Such phase changes can be induced by changes in the operating temperature of the electron gun 140, for example, or changes in the transit time properties of a waveguide (not shown) to the electron gun 140 (such as could be induced by changes in the dielectric gas pressure). Thus, some variable phase may exist between the frequency of the signal supplied by master RF oscillator 105 and the frequency of the electromagnetic field present within the resonant cavity (not shown) of the electron gun 140.
An operator might monitor the output of LINAC 145, then adjust the output of master RF oscillator 105 in an attempt to obtain a desirable output from LINAC 145. However, such observations and adjustments are difficult to make, particularly in real time. Moreover, the quality of output will vary according to the skill of the operator of the system. Thus, such a method and system is not capable of providing desirable outputs on a consistent basis.
Therefore, a need exists for a method and system for easily achieving stable and predictable timing between a laser signal and an electromagnetic field(s) within an accelerator system, whereby a satisfactory output of the accelerator system itself can be easily, inexpensively and reliably obtained.
An improved method and system for synchronizing signals in a particle accelerator system is disclosed, overcoming at least the aforementioned disadvantages.
In one embodiment, a method and system is disclosed whereby phase of laser pulses are monitored, and a high-frequency signal is adjusted as necessary to be substantially in-phase with the laser pulses. In another embodiment a method and system is disclosed whereby a phase of an electromagnetic field in an electron gun is monitored, and a high-frequency signal is adjusted as necessary to be substantially in-phase with the electromagnetic field.